In my opinion, one of the most underutilized tools in the GD&T toolbox is the tangent plane modifier. It was introduced in the 1994 ASME standard, yet some people still think of it as a new concept. Shown in the example below, the tangent plane modifier (circled T) can save money by only controlling the high points of a surface, rather than every point: To understand the drawing above, first realize what parallelism controls if no T modifier is given. Regular parallelism requires that every point on the top of the part be within a tolerance zone of 0.2 mm. This means that regular parallelism inherently controls flatness to the same specification. But there might be times when a designer does not need to control flatness. Perhaps another mating part will contact the top of our part, and we only care about the angle at which the mating part sits. In that case, we don’t need the surface to be flat within 0.2, since our mating part will only feel the high points anyhow. In that case, the tangent plane modifier makes sense. It does not give us a “bonus tolerance” as the MMC modifier does with features of size, but it does have the advantage of being more...
Learn MoreThis time around, I’d like to present another “pet peeve” of mine, at least in the world of GD&T. It involves using the position symbol when the only quality being controlled is perpendicularity. This is very common — it stems from some subconscious notion that if GD&T is going to be used on a hole, it’s got to be the “true position” symbol. NO! Consider the following example. There is a position tolerance applied to the large hole on the left, and the datum being referenced is A. But let’s go to the standard and examine how the geometric control called “position” should be used: ASME Y14.5-2009 (and prior editions) state that position’s main job is to control location — meaning that it involves a distance — and perpendicularity often comes along as part of that position control. Since the large hole given above is already distanced from the edges by plus/minus dimensions, the geometric tolerancing has nothing to do with location. The only relationship that the large hole has with datum A is one of orientation. Therefore, an orientation symbol must be used: Notice the perpendicularity symbol. This is the correct way to identify this hole, since the hole itself now becomes the datum feature...
Learn MoreOccasionally users of GD&T suggest that everything be simplified by just boiling all 14 symbols down to just two or three. (What, you didn’t know there were 14 symbols? Click here for a handy chart!) There is some logic to what these people are saying — namely, that many GD&T symbols overlap others, and position and profile can be used in such a way as to cover the others. But as you might guess, there are pros and cons to this. First, realize that position always controls two qualities: location and orientation. Location is obvious, but don’t forget orientation — because position extends all the way through the depth of a feature, it will control any tilt or angling of that feature. Profile of a surface, if used with datum references and basic dimensions tying it back to those datums, can control all four required qualities: location, orientation, size, and form (shape). Since it covers ALL of these, it can be argued that the other GD&T symbols could be ignored and simply use this one symbol (well, two if you count profile of a line). But there are two problems with this minimalist philosophy: For one thing, it may sometimes be necessary to really only control a particular aspect, such as...
Learn MoreAnother feature that was introduced in the 2009 standard (ASME Y14.5-2009) is the option of creating a “non-uniform” tolerance zone for either of the two profile symbols. Recall that the profile symbols normally specify a uniform boundary or bandwidth that is centered around the “true” or perfect profile. This true profile is first established by basic dimensions on the drawing or by referencing the CAD model, which is the perfect design. Here’s a traditional profile callout: where the tolerance zone looks like: Notice that the 2 mm zone follows the exact contour of the intended design — this is how profile tolerances have always been understood, and will continue to be understood if no other indication is made. But the 2009 version of the Y14.5 standard allows a non-uniform zone, where the feature control frame simply says “non-uniform,” but it is then required that the zone be described in detail on the drawing or by referencing a note or other detailed information. An example: Notice that each side of the tolerance zone has a different radius; the surface of the actual manufactured part can now deviate anywhere within these two curved planes. There may be various reasons why the designer wishes to do this. I...
Learn MoreI wrote about this a long time ago, but it’s worth mentioning again as the new year approaches (for people who still do New Year’s resolutions!). Regular GD&T users should be aware of the certification process for GD&T Professionals. The American Society of Mechanical Engineers (ASME) has established a credential for GD&T proficiency, called GDTP, which stands for ‘Geometric Dimensioning and Tolerancing Professional.’ It is a testing process that measures your ability to interpret and apply tolerances correctly. It is not a license to practice GD&T (no one can legislate that) but it is something that looks nice on your resume! There are actually two certification levels: the Technologist Level, which measures your ability to read GD&T; and the Senior Level, which tests not only interpretation, but also the application of GD&T to a design. One does not necessarily need to become a Technologist first; some people go right for the Senior Level, although they are required to document at least five years of experience in GD&T (and they have a more difficult test!). If you decide to pursue either level, be aware that the questions on the test come from all sections of the 1994 Y14.5 standard. (UPDATE — summer 2017: They now offer the exam based on...
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